1. Field of the Invention
The present invention relates to a read head with a file resettable dual spin valve sensor and more particularly to a dual spin valve sensor in which magnetic spins of first and second pinning layers can be reset with a current pulse from a sense current circuit.
2. Description of the Related Art
A spin valve sensor is employed by a read head for sensing magnetic fields on a moving magnetic medium, such as a rotating magnetic disk. A typical sensor includes a nonmagnetic electrically conductive first spacer layer sandwiched between a ferromagnetic pinned layer and a ferromagnetic free layer. An antiferromagnetic pinning layer interfaces and is exchange coupled to the pinned layer for pinning a magnetic moment of the pinned layer 90xc2x0 to an air bearing surface (ABS) where the ABS is an exposed surface of the sensor that faces the rotating disk. First and second leads are connected to the spin valve sensor for conducting a sense current therethrough. A magnetic moment of the free layer is typically oriented parallel to the ABS in a quiescent condition where the quiescent condition is when the sense current is conducted through the sensor in the absence of any signal fields. The magnetic moment of the free layer is free to rotate from the parallel position in response to signal fields from the rotating magnetic disk.
The thickness of the spacer layer is chosen so that shunting of the sense current and a magnetic coupling between the free and pinned layers are minimized. This thickness is typically less than the mean free path of electrons conducted through the sensor. With this arrangement, a portion of the conduction electrons are scattered at the interfaces of the spacer layer with respect to the pinned layer and the free layer. When the magnetic moments of the pinned and free layers are parallel with respect to one another scattering is minimal and when their magnetic moments are antiparallel scattering is maximized. Changes in scattering, in response to field signals from a rotating disk, changes the resistance of the spin valve sensor as a function of cos xcex8, where xcex8 is the angle between the magnetic moments of the pinned and free layers. The sensitivity of the sensor is quantified as magnetoresistive coefficient dr/R where dr is the change in resistance of the sensor between parallel and antiparallel orientations of the pinned and free layers and R is the resistance of the sensor when the moments are parallel.
The transfer curve (readback signal of the spin valve head versus applied signal from the magnetic disk) of a spin valve sensor is a substantially linear portion of the aforementioned function of cos xcex8. The greater this angle, the greater the resistance of the spin valve to the sense current and the greater the readback signal (voltage sensed by processing circuitry). With positive and negative signal fields from a rotating magnetic disk (assumed to be equal in magnitude), it is important that positive and negative changes of the resistance of the spin valve sensor be equal in order that the positive and negative magnitudes of the readback signals are equal. When this occurs a bias point on the transfer curve is considered to be zero and is located midway between the maximum positive and negative readback signals. When the direction of the magnetic moment of the free layer is parallel to the ABS in the quiescent state the bias point is located at zero and the positive and negative readback signals are equal when sensing positive and negative signal fields from the magnetic disk. The readback signals are then referred to in the art as having symmetry about the zero bias point. When the readback signals are not equal the readback signals are asymmetric which equates to reduced storage capacity of a magnetic disk drive.
The location of the bias point on the transfer curve is influenced by three major forces on the free layer, namely a demagnetization field (HD) from the pinned layer, a ferromagnetic coupling field (HF) between the pinned layer and the free layer, and sense current fields (Hl) from all conductive layers of the spin valve except the free layer. When the sense current is conducted through the spin valve sensor, the pinning layer (if conductive), the pinned layer and the first spacer layer, which are all on one side of the free layer, impose sense current fields on the free layer that rotate the magnetic moment of the free layer in a first direction. The ferromagnetic coupling field from the pinned layer further rotates the magnetic moment of the free layer in the first direction. The demagnetization field from the pinned layer on the free layer rotates the magnetic moment of the free layer in a second direction opposite to the first direction. Accordingly, the demagnetization field opposes the sense current and ferromagnetic coupling fields and can be used for counterbalancing.
A dual spin valve sensor employs a ferromagnetic free layer between first and second ferromagnetic pinned layers wherein a first spacer layer separates the first pinned layer from the free layer and a second spacer layer separates the second pinned layer from the free layer. The first and second pinned layers are pinned by first and second antiferromagnetic pinning layers. With this arrangement, the magnetoresistive coefficient is increased by a factor of approximately 1.4 due to the spin valve effect on each side of the free layer. The magnetic spins of the pinning layers are set by cooling from a high temperature in the presence of a magnetic field. In a standard dual spin valve, it is necessary that the magnetic spins of the first and second pinning layers be set parallel with respect to each other in order for the spin valve effect to be additive. This is the case of a simple free layer which has only one layer or layered structure. A problem with this design arises when the dual spin valve undergoes an in file reset. The current in the sensor applies a magnetic circumferential magnetic field to the sensor. This results in the magnetic field applied to the two pinned layers being in opposite directions, setting the moments antiparallel. The spin valve effect from the upper and lower portions of the spin valve are no longer additive but instead subtract. Thus, no signal is produced in this configuration for a spin valve with a simple free layer.
Over the years a significant amount of research has been conducted to improve the magnetoresistive coefficient dr/R of spin valve sensors without adversely affecting other performance factors such as biasing of the free layer and thermal stability of the pinning layers. These efforts have increased the storage capacity of computers from kilobytes to megabytes to gigabytes.
We have provided a dual spin valve sensor wherein first and second antiferromagnetic pinning layers of the sensor can be reset in a magnetic disk drive by a current pulse through the sense current circuit, which resetting is referred to hereinafter as file resettable. This has been accomplished by providing an antiparallel (AP) coupled free layer structure which is located between the first and second pinned layers with a first nonmagnetic conductive spacer layer between the first pinned layer and the free layer structure and a second nonmagnetic conductive spacer layer between the second pinned layer and the free layer structure. First and second antiferromagnetic pinning layers are exchange coupled to the first and second pinned layers for the purpose of setting the magnetic moments of the pinned layers. The file reset pulse sets these moments antiparallel with respect to each other, which is the proper orientation so that the spin valve effects, one on each side of the AP free layer structure, are additive. The free layer structure includes a nonmagnetic conductive antiparallel (AP) coupling layer which is located between first and second antiparallel (AP) coupled free layers. It is necessary that one of the first and second AP coupled free layers be thicker than the other of the first and second AP coupled free layers so that when the free layer structure responds to a signal field from a rotating magnetic disk one of the AP coupled free layers is responsive to the signal field from the disk and controls the rotation of the free layer structure. Since the thinner AP coupled free layer is strongly antiparallel coupled to the thicker AP coupled free layer it will rotate in the same direction as the thicker AP coupled free layer even though its magnetic moment is antiparallel to the magnetic moment of the thicker AP coupled free layer. Accordingly, when a signal field rotates the AP coupled free layer structure it is important that the orientations of the magnetic moments of the first and second pinned layer structures be in phase with respect to the orientations of the magnetic moments of the free layer structure so that the spin valve effects, one on each side of the free layer structure, are additive to promote a high magnetoresistive coefficient dr/R. In order for this in-phase relationship to occur it is necessary that the magnetic moments of the first and second pinned layer structures be antiparallel with respect to one another and that they be pinned in these positions by the first and second pinning layers. Accordingly, the magnetic spins of the first and second pinning layers must be properly set to accomplish this purpose.
A current pulse conducted through the conductive layers of the spin valve sensor via the sense current circuit will cause magnetic fields on the first and second pinned layer structures from other conductive layers of the sensor. These fields set the magnetic spins of the first and second pinning layers. A current pulse sufficient to raise the temperature of the spin valve sensor to the blocking temperature of the materials employed for the first and second pinning layers will provide the necessary heat so that the current fields from the conductive layers of the sensor will implement a proper setting of the magnetic spins of the pinning layers. For example, iron manganese (FeMn) has a blocking temperature of 160xc2x0 C. With a current pulse through the sense current circuit of a magnitude sufficient to raise the temperature of the spin valve sensor at or above 160xc2x0 C. current fields from the conductive layers of the spin valve sensor on one side of the first pinned layer structure will orient the magnetic moment of the first pinned layer structure in a first direction perpendicular to the ABS while current fields from the conductive layers of the spin valve sensor on one side of the second pinned layer will cause the magnetic moment of the second pinned layer to be antiparallel to the magnetic moment of the first pinned layer. This will cause the magnetic spins of the first and second pinning layers to align with the magnetic moments of the first and second pinned layers which means that the magnetic spins of the first and second pinning layers are also antiparallel. When the current pulse is terminated and the spin valve sensor cools below the blocking temperature the magnetic spins of the first and second pinning layers are set in their orientations since they are no longer free to move at a temperature below their blocking temperatures.
In a preferred embodiment the same material is employed for each of the first and second pinning layers with a blocking temperature preferably below 280xc2x0 C. It should be noted that the operating temperature of a spin valve sensor in a magnetic disk drive is between 80xc2x0 C. to 120xc2x0 C. with a sense current pulse of approximately 0.03 volts. We have found that a current pulse of approximately 0.9 volts for a period of 10 nanoseconds is sufficient for resetting pinning layers with a low blocking temperature. It should be noted that the key to providing a file resettable dual spin valve sensor is the AP coupled free layer structure wherein the magnetic moments of the first and second AP coupled free layers are antiparallel with respect to one another. Since the current fields on each side of the free layer structure are likewise antiparallel this then implements the required in-phase relationship between the first and second pinned layer structures and the AP coupled free layer structure.
Employment of the AP coupled free layer structure has another distinct advantage. In high recording densities of the future the prior art single free layer may be required to be as thin as 20 xc3x85 of nickel iron (NiFe) in order to match the low moment of the signal fields from a rotating magnetic disk. Unfortunately, this thickness is too thin to provide optimized magnetoresistance between the free and pinned layers. The thickness of the free layer that optimizes the magnetoresistive signal is largely governed by the longer of the spin-up and spin-down electron mean free paths within the ferromagnetic layers which is typically about 50 xc3x85. Layers thinner than the optimal thickness do not permit electrons to travel as far as if the layer thickness were optimized thereby reducing the magnetoresistance. With the AP coupled free layer structure it is only necessary that the net magnetic moment of the free layer structure be matched to the moment of the signal field. Accordingly, in the dual spin valve sensor each of the AP coupled free layers has a thickness sufficient to optimize the magnetoresistance on each side of the free layer structure. For instance, the first AP coupled free layer may be 50 xc3x85 of nickel iron (NiFe) and the second AP coupled free layer may be 70 xc3x85 of nickel iron (NiFe). This provides the optimized magnetoresistance on each side of the free layer structure while providing a net moment of the free layer structure of only 20 xc3x85 of nickel iron (NiFe) which can match future requirements of low moment signals from the rotating magnetic disk. An AP coupled free layer structure is fully described in commonly assigned U.S. Pat. No. 5,768,069 which is incorporated by reference herein.
In a further preferred embodiment, each of the first and second pinned layer structures is an antiparallel (AP) pinned layer structure with a nonmagnetic conductive antiparallel (AP) coupling layer between first and second antiparallel (AP) ferromagnetic pinned layers. The AP coupled pinned layer structure is fully described in commonly assigned U.S. Pat. No. 5,465,185 which is incorporated by reference herein. The first and second pinning layers pin the first AP coupled pinned layers of the first and second AP pinned layer structures perpendicular to the ABS in an antiparallel relationship and the first AP pinned layers of the first and second AP pinned layer structures pin the second AP pinned layers of the first and second AP pinned layer structures perpendicular to the ABS antiparallel with respect to one another. Accordingly, the desired in-phase relationship between the first and second AP pinned layers and the free layer structure is obtained by the aforementioned process of setting the magnetic spins of the first and second pinning layers. An advantage of the AP pinned structures is that the net moment of these structures may be less than that of a single pinned layer, promoting a higher exchange field when these structures are coupled to the antiferromagnetic pinning layers. Further, since the magnetic moments of the first and second AP pinned layers of each of the first and second AP pinned layer structures are antiparallel there is flux closure between these AP pinned layers.
Since one of the AP pinned layers has to be thicker than the other in order to control a setting of the magnetic spins of the corresponding pinning layer, each AP pinned layer structure has a net demagnetization field. In a preferred embodiment the first and second AP pinned layer structures are symmetrical so that their net demagnetization fields are equal. Since these net demagnetization fields are in opposite directions they will completely counterbalance one another so as to exert no net demagnetization field on the free layer structure. Accordingly, demagnetization fields from the first and second AP pinned layer structures will have no influence on rotating the magnetic moments of the free layer structure from their parallel orientations to the ABS which is required for a zero bias point and read signal symmetry. It should be noted that if the first and second AP pinned layer structures are symmetrical sense current fields from layers of the first and second AP pinned structures on the free layer structure will likewise counterbalance each other. Optionally, the thicknesses of the layers of the first and second AP pinned layer structures may be fashioned with different thicknesses so as to provide a net demagnetization field and a net sense current field on the free layer structure which may be employed for counterbalancing ferromagnetic coupling fields exerted on the first and second AP coupled free layers by the second AP pinned layers of the first and second AP pinned layer structures. The first and second AP pinned layer structures provide considerable flexibility in maintaining a desired parallel position of magnetic moments of the free layer structure in a quiescent condition of the sensor, namely conduction of sense current in the absence of any signal fields.
An object of the present invention is to provide a read head with a file resettable dual spin valve sensor.
Another object is to provide a spin valve sensor with a free layer structure which optimizes magnetoresistance and moment matching of the media.
A further object is to provide a dual spin valve sensor which promotes symmetry of a read signal, which can be reset in a magnetic disk drive and which promotes high magnetoresistance and recording densities.
Other objects and attendant advantages of the invention will be appreciated upon reading the following description taken together with the accompanying drawings.